FIG. 1 shows the transmission of data in a wireless system according to the state of the art. Several transceivers belonging to the same wireless local area network (WLAN) use the same data transmission channel by means of time sharing. At any specific time only one transceiver is transmitting. Accordingly the transmissions from each transceiver are burst like. For helping the receiving transceiver to identify a data transmission burst and for extracting the delivered information data the transmitting transceiver sends a predefined preamble signal preceding the data portion of the data transmission burst. The transceiver that receives the data transmission burst comprises a preamble detection unit that identifies the preamble and thus identifies the data transmission burst. The transceiver uses further the preamble for estimating data transmission and channel parameters such as channel response and carrier and timing offsets that are needed for the data information extraction.
Commonly several communication networks share the same data transmission media. Specifically collocated wireless networks utilize the same frequency spectrum.
FIG. 2 shows two collocated wireless networks according to the state of the art.
Wireless local areas networks (WLAN) represent a new form of communications among personal computers or other devices that wish to deliver digital data. A wireless network is one that does not rely on cable as the communications medium. Whether twisted pair, coax, or optical fibres, hard wiring for data communication systems within a building environment is expensive and troublesome to install, maintain and to change. To avoid these disadvantages wireless networks transmit data over the air using signals that cover a broad frequency range from few MHz to a few terahertz. Depending on the frequency involved wireless networks comprise radio wireless networks, microwave wireless networks and infrared wireless networks.
Wireless networks are used mainly for connecting devices within a building or connecting portable or mobile devices to a network. Further applications are keeping mobile devices in contact with a data base and ad hoc networks for example in committee or business meetings.
Wireless local area networks (WLAN) and wireless personal area networks (WPAN) are used to convey information over relatively short ranges. A wireless personal area network (WPAN) is defined in the IEEE 802.15.3 standard.
In many situations and scenarios several wireless local area networks (WLANs) are operated simultaneously with each other in the same local area. A typical situation would be a big office wherein many office cubicles are located belonging to different divisions of the same company, e.g. search division, accounting division, marketing division. The computers of each division are connected in such a situation by means of separate wireless local area networks (WLANs). A wireless local area network (WLAN) comprising several transceivers is referred to as a Piconet.
FIG. 2 shows typical scenario where two wireless local area networks (WLANS) are operated in the same local area.
In the shown example a first transmitting transceiver A2 transmits data to a receiving transceiver A4 of the first wireless local area network WLANA on the data transmission channel of the wireless local area network WLANA. Further a transmitting transceiver B3 of the second wireless local area network WLANB transmits data to a receiving transceiver B1 of the same wireless local network WLANB on the data transmission channel of this wireless local area network. The data exchange between transceivers is performed half duplex, i.e. a transceiver can either send or receive data over a data link to another transceiver of the same wireless local area network. The data are exchanged via data packets.
Each Piconet WLANi has its respective data transmission channel, i.e. the data transmission channel is used by all transceivers of the corresponding Piconet WLANi.
In most cases the frequency resources available for a wireless local area network WLAN are bounded by regulations. Usually a certain frequency band is allocated for the wireless local networks. Within this frequency band each transceiver is required to radiate no more than a specified average power spectral density (PSD).
To operate several wireless local area networks simultaneously several proposals have been made.
In frequency division multiplexing (FDM) systems according to the state of the art the allocated frequency band is divided into several sub-frequency bands. In FDM-system each data transmission channel and consequently each Piconet is using a different frequency sub-band. Thus, data transmission in different Piconets (WLANs) can simultaneously be performed without interference.
The disadvantage of FDM-systems is that the available capacity for each Piconet is reduced compared to the case where any Piconet is allowed to use the entire allocated frequency band.
The channel capacity is given by the following formula:
  cap  =      ∫                  log        ⁡                  (                      1            +                                          PSD                ⁡                                  (                  f                  )                                                            N                ⁡                                  (                  f                  )                                                              )                    ⁢              ⅆ        f            
The capacity of each Piconet is larger if it will be allowed to use the full frequency band instead of just the allocated frequency sub-band. The reduction in the capacity in FDM-systems translates directly to throughput reduction Consequently the achievable data bit rate for any specific transmitter-receiver distance is reduced in FDM-systems.
In a CDMA-DSSS (Code Division Multiple Access—Direct Sequence Spread Spectrum) system according to the state of the art a direct sequence spread spectrum is used as a modulation scheme. In DSSS a sequence of many short data symbols is transmitted for each information symbol. In order to support several data transmission channels or Piconets different data sequences with low cross correlation between them are used for different data transmission channels.
In a CDMA-DSSS-system each channel can use the entire frequency band until the maximum possible throughput can be achieved. If some Piconets are working in the same area then the transmission of one Piconet is seen as additional noise by the other Piconets.
The disadvantage of the CDMA-DSSS-System is that there exists a so (called near-far problem. When a transceiver in one Piconet is transmitting this transmission will be seen as additional noise by other Piconets. The level of the additional noise is proportional to the cross correlation between the spreading sequences and the received power level of the interferer's signal. For example if the interfering transceiver of Piconet A is close to a receiving transceiver of Piconet B, i.e. closer than a transmitting receiver of Piconet B then the added noise level that the receiving transceiver of Piconet B sees causes a significant reduction in the achievable bit rate for the receiver, so that even a complete blocking of the data transmission channel can occur.
A further proposal according to the state of the art to operate several wireless local area networks (WLANs) simultaneously is to use a CDMA-FH(Code Division Multiple Access—Frequency Hopping)—System. In this CDMA-FH-System the original frequency band is divided into several sub-frequency bands. Any transmitting transceiver uses a certain frequency sub-band for a certain time interval and moves then to the next frequency band. A predefined frequency hopping sequence controls the order of sub-frequency bands such that both the transmitting and receiving transceiver has the information when to switch to the next frequency and to what sub-frequency band.
In a conventional CDMA-FH-System the different data transmission channels are assigned with different frequency hopping sequences.
FIG. 3 shows a CDMA-FH-System according to the state of the art with data transmission channels A CDMA-FH-System with four data transmission channels can operate four Piconets or wireless local area networks (WLANs) simultaneously at the same local area. In the shown example any transceiver uses a certain frequency band for a transmission interval for 242 ns, remains idle for a predetermined guard time of 70 ns and uses the next frequency band within the next transmission interval etc.
The frequency hopping sequence is fixed for any data transmission channel A, B, C, D. In the given example data transmission channel A has the frequency hopping sequence abc, channel B has the frequency hopping sequence acb, channel C has the frequency hopping sequence aabbcc and channel D has the frequency hopping sequence aaccbb.
A collision is a situation when two transceivers use the same frequency band at the same time. For example a collision between data transmission channel A and data transmission channel B occurs during the first transmission interval when both channels A, B use frequency fa and during the fourth transmission interval when both channels A, B use again frequency fa. A further collision is for example between channel B ant channel D during the first transmission interval when both channels B, D use frequency a and the sixth transmission interval when both channels B, D use frequency fb.
When the frequency hopping order of two wireless networks differs two transceivers that belong to different wireless local area networks can transmit at the same time. It may happen that both transceivers use the same carrier frequency at the same time.
FIG. 4 illustrates simultaneous data transmission from transceivers in two different networks WLAN A, WLAN B at the same time. Each data transmission burst comprises a preamble signal and a data signal. The data signal includes header data and payload data.
The preamble detector in a transceiver that intends to decode bursts in network A needs to discriminate preambles for network B. Further the receiving transceiver is able to detect and estimate relevant parameters from a legitimate preamble in the potential simultaneous presence of other transmissions.
One possible CDMA-FH solution is based on OFDM and is called Multiband OFDM. In this case the transceiver transmits a single OFDM in one band and then hops to the next band for transmitting the next OFDM symbol. FIG. 3A depicts 6 OFDM symbols for each channel.
As shown in FIG. 3A the Multiband OFDM Transceiver performs in a time frequency interleaving (TFI) mode band-hopping wherein in each frequency band an OFDM symbol is transmitted. The band-hopping sequence is defined by a TFC code (Time frequency code) stored in a memory. Different collocated networks use different TFC codes. This enables simultaneous transmission of different networks. OFDM symbols from collocated networks collide. In common scenarios the collision level enables efficient communication. Yet in some cases the collisions situation is severe and the communication is not efficient. To overcome severe collisions between transmission of different networks frequency domain separation (known as FDM) between the wireless networks can be implemented. This is achieved by adding TFC codes with constant band usage (fixed frequency bands). Accordingly a Multiband OFDM Transceiver according to the state of the art is switchable between a time frequency interleaving mode (TFI mode) and a fixed frequency interleaving mode (FFI mode). FIG. 3B shows 7 channels (7 TFC) where 4 channels are of TFI type and 3 channels are of FFI type.
As can be seen in FIG. 3 the transceiver occupies in the TFI mode three frequency bands, wherein each frequency band has a predetermined frequency bandwidth. In an evolving multi-band OFDM standard which defines the physical layer of an ultra wideband based wireless personal access network the tranceiver transmits in both modes, i.e. in the TFI mode and in the FFI mode the same preamble as shown in FIG. 5. FIG. 5 shows a so called long preamble having N=24+6 OFDM symbols. The preamble according to the state of the art as shows in FIG. 5 is subdivided-into two sections. The first preamble section (24 OFDM symbols) is typically used for packet detection/acquisition, coarse frequency estimation, gain control, synchronization, timing and offset estimation, The first section of the conventional preamble as shown in FIG. 5 comprises 24 OFDM symbols, wherein the first twenty-one OFDM symbols are identical and the last three OFDM symbols are inverted. These three OFDM symbols are used for synchronization within a data packet. The second preamble section is typically used for channel estimation, wherein the channel estimation sequence comprises six OFDM symbols. According to the evolving multiband OFDM standard the period of one OFDM symbol is 312.5 nSec, i.e. a data length of 242.5 nsec (128 samples at 528 Msps) and a silence time of 70 nSec (37 samples at 528 Msps) between two transmissions. Consequently the OFDM symbol rate Rs=3.2 MHz=1/312.5 nSec. When using three frequency bands there are seven possible time frequency codes (TFC). The first four TFC codes define the frequency band hopping sequence when the transceiver is in the TFI mode. When the transceiver is switched to the FFI mode the transceiver transmits the signal in a fixed frequency band. As shown in the following table and in FIG. 3A, the fifth TFC code indicates that transceiver transmits a signal in a first frequency band, the sixth TFC code indicates that the transceiver transmits the signal in a second frequency band and the seventh TFC code indicates that the transceiver transmits a signal in a third frequency band.
The following TFC code have three frequency bands as summarized in the following table:
TABLE 1TFC IndexCodeType1[1, 2, 3]TFI2[1, 3, 2]TFI3[1, 1, 2, 2, 3, 3]TFI4[1, 1, 3, 3, 2, 2]TFI5[1]FFI6[2]FFI7[3]FFI
The multiband frequency division multiplexing transceiver according to the prior art uses two types of preambles. The first type of the first section of the preamble as shown in FIG. 5 includes N=24 OFDM symbols and is called a long preamble. The second preamble is called a short preamble and comprises N=12 OFDM symbols in the first preamble section.
The short preamble is used when a sequence of packets is transmitted by the same transmitter in scenarios where the detection is relatively easy to perform, i. e. when a good signal to noise ratio is given and the inter-packet guard time is known.
Within the first section of the preamble, which includes N OFDM symbol, each OFDM symbol is typically not a frequency domain signal. The symbol is a time domain signal which is defined by a time domain sequence. Nevertheless these symbols maintain the same duration and power as the OFDM symbols and are referred in this document as OFDM symbols.
In the conventional OFDM transceivers for wireless networks the preambles are identical both in the FEI mode and in the TFI mode. The beginning of the preamble is shown in FIG. 5, i.e. wherein the first N-K symbols are used for preamble detection by the receiving transceiver. The section of the K=3 preamble OFDM symbols which is also called a delimiter is provided for the identification of the end of the first section of the preamble. The OFDM symbols of the frame synchronization sequence as shown in FIG. 5 are inverted with respect to the OFDM symbols of the first 21 OFDM symbols. An OFDM symbol inversion enables an identification of the delimiter at the receiving side by watching a single frequency band out of the three frequency bands.
However the OFDM transceiver according to the state of the art using the same preamble sequence in the TFI mode and in the FFI mode has two major disadvantages. Since the period of one OFDM symbol within the packet synchronization sequence as shown in FIG. 5 having 21 OFDM symbols is 312.5 nSec there is a ripple in the frequency domain with frequency peaks every 3.2 MHz. The rippled power spectrum of the preamble affects the overall measured transmission frequency spectrum that includes the preamble and the data. The effect of the rippled preamble spectrum is in particular severe in the FFI mode of the transceiver because the preamble signal includes a long section of repeated OFDM symbols in the same frequency band at a periodicity rate RS=3.2 MHz. For maximizing the transceiver performance the maximum allowed transmitted power is used. Regulation bodies such as the Federal Communication Commission (FCC) in the USA limit the PSD in a resolution band width RPSD=1 MHz. This means that within any 1 Mz band the power is limited. Since the PSD of the state of the art FFI node has ripple, the transceiver reduces the overall transmitted power, compared to a scenario where the PSD is flat, such that the spectral peaks do not exceed the allowed level set by the laws. The PSD defined by the FCC for ultra-wideband and communication is for instance −41.3 dBm/MHz. Actually the reduced transmission power of the transceiver results in a reduced signal to noise ratio (SNR) at the receiving transceiver so that the performance of the data transmission is degraded. The necessary transmission power reduction results in a reduced data rate, reduced supported range and an increased bit error rate BER.
A further draw back for conventional transceiver according to the state of the art using the same preamble in the TFI mode and in the FFI mode is that the detection of the delimiter in the FFI mode is uncertain. The delimiter shown in FIG. 5 comprises 3 OFDM symbols, i.e. an OFDM symbol for each frequency band used in the TFI mode for frequency hopping. The detection of the delimiter is essential for the next stages of the packet reception. This is done by detecting the last K=3 OFDM symbols in the first section of the preamble. In the TFI mode a single inverted OFDM symbol within the delimiter uniquely identifies the timing of the delimiter. However in the FFI mode a fixed frequency is used and a lost symbol at the receiving transceiver e.g. caused by a colliding symbol, may cause uncertainty in the delimiter timing of the receiving transceiver. For example if the OFDM symbol N−2 is badly received the delimiter detector in the receiving transceiver might mistakenly assume that the OFDM symbol N−1 is symbol N−2 and accordingly output a wrong decision. Under a scenario of simultaneously operating networks this situation might cause a packet loss since some of the symbols may be collided and wrongly detected.
Accordingly it is the object of the present invention to provide a preamble generator for a multiband orthogonal frequency division multiplexing transceiver which overcomes the above mentioned drawbacks and in particular to provide a preamble generator wherein the preamble spectrum in an FFI mode is more flat.